The proliferation of carrier system service throughout the telecommunications industry has mandated the need for testing and conditioning an ever increasing number of telephone lines of digital loop carrier or pair gain systems. Traditionally, such testing has been carried out by means of a metallic DC bypass pair, which extends from the central office terminal to the remote terminal where access to the telephone lines of interest is provided. Equipment installed at the central office transmits test condition signals over the DC bypass pair to the remote terminal and subsequently to the line under test (LUT).
Unfortunately, this test and conditioning scheme suffers from a number of shortcomings including the fact that, in may cases, the test results represent not only conditions and faults on the line (loop) being tested, but also those of the DC bypass pair, which is connected in series with the LUT. Consequently, fault conditions such as noise, crosstalk, unacceptable voltage levels, etc. on the DC bypass pair manifest themselves by way of inaccurate results in the course of evaluating the LUT.
Another problem is the fact that such test systems suffer from an inherent functional limit of approximately 2800 ohms (roughly ten miles of copper wire), beyond which point line testing is generally considered to be unreliable. Thus, the effective reach of a conventional telephone line test system is limited by the distance between the central office and the remote terminal. Indeed, in many instances this distance is so great as to preclude the use of a centralized test system which utilizes the DC bypass metallic pair for loop carrier applications. Moreover, with the advent and proliferation of optical fiber as a communication link for voice and data traffic between the central office and the remote terminal, it can be seen that the ability to use a metallic bypass pair for test purposes is becoming more limited. In some cases metallic bypass pairs are not even included with fiber optic installations, making the use of a conventional centralized test system virtually impossible.